A mixed solvent, obtained by mixing a single solvent for dissolving a specific substance with another kind of additional solvent, is changed in solubility characteristics compared to a single solvent. Furthermore, as the amount of the additional solvent increases, solubility of the mixed solvent becomes significantly different from that of the single solvent. Since a mixed solvent having solubility different from a single solvent is also different in solubility from a substance to be dissolved, for example, a polymer, it is typically regarded as impossible to be used for dissolving a polymer, instead of a single solvent.
Requirements for using a mixed solvent instead of a single solvent for dissolving a polymer are as follows: (1) a mixed solvent should have solubility similar to that of a single solvent so as to dissolve a polymer, and (2) a mixed solvent should contain an additional solvent in a maximum amount so that the amount of a single solvent is minimized. However, as the amount of the additional solvent is larger, a solubility difference between the mixed solvent and the single solvent may increase, and requirements (1) and (2) may not be simultaneously satisfied.
Meanwhile, in order to determine solubility or miscibility of substances, similarity of the substances should be compared using unique properties thereof. Among a variety of unique properties that affect solubility or miscibility, particularly useful is a solubility parameter for quantifying the extent of interaction of substances. Specifically, individual substances have unique solubility parameters, and substances having similar solubility parameters are dissolved in or are miscible with each other.
Among solubility parameters proposed and utilized based on various theories or concepts, the Hansen solubility parameter (HSP) devised by Dr. C. Hansen, 1967, is known to evaluate solubility characteristics very accurately. As for HSP, the extent of interaction of substances is considered by the following three elements:
(1) δD is a nonpolar solubility parameter owing to dispersion interactions;
(2) δP is a polar solubility parameter owing to permanent dipole-permanent dipole interactions; and
(3) δH is a hydrogen bond solubility parameter.
Thus, HSP is widely utilized to more accurately and systematically evaluate solubility or miscibility of the substances because it provides specific interaction information of the substances, compared to the other solubility parameters.HSP=(δD,δP,δH),(J/cm3)1/2  (1)δTot=(δD2+δP2+δH2)1/2,(J/cm3)1/2  (2)
HSP is regarded as a vector having a magnitude and direction in a space made up of three elements, and δTot shows the magnitude of HSP vector. A basic unit of the HSP is (J/cm3)1/2. Such HSP values are calculated using a program referred to as HSPiP (Hansen Solubility Parameters in Practice) by a research group led by Dr. Hansen, who developed HSP. As mentioned above, when two substances have similar HSP values, they dissolve well in each other. In order to determine that substances are similar, three HSP elements and the HSP magnitude of individual substances should be similar because HSP is a vector. All the substances are compared and analyzed for the similarity difference based on unique HSPs thereof, and thereby whether substances of interest may be dissolved in each other may be estimated.
Although whether pure substances are dissolved well in each other may be estimated via HSP similarity analysis of individual pure substances, a mixture comprising two or more kinds of pure substances may greatly vary in solubility depending on the composition thereof. The mixed solvent may be efficiently employed so as to be adapted for various purposes so long as solubility properties thereof are appropriately utilized. In particular, such a mixed solvent may play a great role in replacing a conventionally used single solvent or adjusting specific properties. For example, as shown in the flowchart of FIG. 1, when a great cost saving effect may be created under the condition that an expensive single solvent having high polymer solubility for use in polymer processing is replaced with another inexpensive single solvent, a desired purpose may be achieved in the presence of a replaceable single solvent, but solutions for cost savings have not yet been proposed in the absence of such a replaceable solvent.
Even if there is not provided a single solvent able to completely replace a conventional single solvent, cost savings may be ensured so long as the amount of a single solvent is maximally reduced, and thus solutions therefor are required.
Since the mixed solvent significantly varies in solubility characteristics depending on the composition thereof as mentioned above, the use of a mixed solvent may be considered as an alternative to reducing the amount of a single solvent. Although a variety of mixed solvents may be prepared depending on the kind of solvent used, the use of a mixed solvent made by adding another additional solvent that is inexpensive compared to a conventional single solvent may be taken into consideration.
Conditions necessary for using a mixed solvent in lieu of a single solvent are that a mixed solvent should have solubility similar to that of a single solvent so as to dissolve a target substance without particular problems and also that a mixed solvent should contain a single solvent in a minimum amount so as to maximize cost saving effects. However, the mixed solvent obtained by mixing a single solvent with another additional solvent is problematic because the solubility thereof varies. Therefore, there is a need to study a method of searching for a mixed solvent composition having solubility similar to that of a single solvent while maximally reducing the amount of a single solvent by properly adjusting the solubility of the mixed solvent.